Elimination, types

Depolymerization of starch in alkaline solution proceeds more slowly than in acid and produces isosaccharinic acid derivatives rather than D-glucose as a major product. The mechanism involves a -elimination-type reaction (48). 

It should be noted that the cyano group is lost by a retro-Claisen-type fragmentation 6->7 which gives automatically the pyrrocorphin 7 on the hexahydro oxidation level rather than by a /i-elimination-type process which would give a tetrahydroporphyrin. A similar fragmentation process has been observed in the total syntheses of chlorophyll a (sec Section 1.2.1.) and of a tolyporphin model (see Section 1.3). 

Bromo-4-chloro-lH-pyrazolo[3,4-d]pyrimidine could be easily fimc-tionalized at C-3 and C-4 in a one-pot two-step microwave-assisted process (Scheme 34) [55]. Ding and Schultz reported that nucleophilic substitution of the addition-elimination type at the C-4 position with amines and anilines smoothly occurred under acidic conditions in dioxane upon irradiation... 

Cytostatic drugs for which inhibitors are primarily needed are often not only substrates of P-gp but also inducers of P-gp overexpression with one or even several type II units. An important point to clarify would be whether or not type II units are required to achieve a cytostatic effect. If they were not required, it might be possible to eliminate the P-gp-inducing effect that causes multidrug resistance by eliminating type II units. So far, the only other possibility of overcoming multidrug resistance is to inhibit the transporters involved. Inhibition can be achieved essentially by two different concepts ... 

In this context, an avalanche of studies were devoted to acid-base reactions in their broadest sense (i.e., the Lewis picture), also involving complexation reactions, to the typical organic reactions of addition, substitution, and elimination types, involving nucleophilic and electrophilic reagents including the case of radicalar reactions and excited states (for a review see Ref. [11]) in which our group has... 

While no such eliminative-type reaction of an alkyl or aralkyl thiolsulfonate with a nucleophile has apparently yet been reported, it seems almost certain that if one were to look at thiolsulfonates where the hydrogen on the carbon adjacent to the sulfenyl sulfur was sufficiently acidic that examples of the kind of reaction would be found. 

In the first step, 01 attacks P9 and displaces Cl 10. After deprotonation of N3, a carbocation at C2 (stabilized by resonance with N4) is formed. Addition-elimination then gives the product. An alternative and reasonable mechanism would have C7 attack C2 before the C2-01 bond cleaves (addition-elimination type mechanism), but the conventional wisdom is that the reaction proceeds through the nitrlium ion intermediate. 

Electrophilic halogenation processes can involve conventional mechanisms with a carbocation intermediate, or pathways that are of the addition-elimination type. Sometimes the process does not go beyond the addition stage there are also examples of halodehalogenation in which the displaced halogen attaches elsewhere in the ring, as in the chlorination of 3-bromobenzo[fe]thiophene to give 2-bromo-3-chlorobenzo[6]thiophene as one of the products (73IJS233). 

G. Decomposition of N-Nitrosohydroxylamines (Free Acid) (Elimination-Type Reaction)... 

Eliminator-Type Packing Okubo, M.jHirai, E. Hayashi, Y. 

Determine which nucleophilic aromatic substitutions are likely, and propose mechanisms for both the addition-elimination type and the benzyne type. Problems 17-52, 61, 62, G3, and GG... 

The conversion of one cyclopropenium ion into another, a substitution reaction, is gaining synthetic importance. Until recently, only nucleophilic substitution reactions of the nucleophilic addition-elimination type were known. However, three other processes are now recognized. These processes are ... 

The evidence, to be discussed below, very strongly favors an addition-elimination type of mechanism. For vinyl exchange, the scheme is... 

Even we have used weaker conditions than Jones s reactant, the oxidation of product 13h is always coupled, working with greater substrate quantities than few milligrams, with the total insaturation of ring obtaining compound 14. Probably this happens because of the elimination type 1,4 of a water molecule, favoured by the acidic environment, according with the mechanism represented in Scheme R, section A [55]. 

Lin and coworkers used LiBr in acetonitrile to carry out the conversion of the frawj-diepoxide (313) to (314) in this instance the enol(ate) generated after the initial aldehyde formation causes aromatization and opening of the other epoxide in an elimination-type reaction (equation 133). The use of BF3 in ether allowed isolation of the dialdehyde (315). It is worth noting that the more acidic conditions cause epoxide rearrangement to occur more rapidly than enolization reactions of this sensitive substrate. 

Double bonds can also migrate transannularly if the conformative requirements are fulfilled. Characteristic examples are the [1,2,(3)7]-, and [l,2,(3)8]-eliminations of 17291) and 7 73 98) to give the bicyclic products 156 (12%) and 174 (14%). The [l,2,(3)5]-elimination type can be realized with cycloalkenes that have a suitable side chain. These complex eliminations succeed thermally, under the influence of bases,... 

Cyclic exponents of the same elimination type are of particular interest. Thus, numerous cyclopentanones photolytically decarbonylate to give 1,4-dienes (p. 876 ff. in Ref. 108)). With thujone (194) this [l,2,(3)4]-elimination of carbon monoxide proceeds quantitatively to give 195 109). With silver nitrate the norcaradiene 196 yields 197 (95%)110) apparently regioselectively, and the [l,2,(3)4]-elimination of methanethiol from 198 to give 199 was realized thermally (16%), acid-catalyzed (59%), and photochemiciiily (ca. 5%)105). For the acid-catalyzed reaction (acetic acid, 100 °C) a non-stereospecific process has been proved 105). 

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